1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, in electrophotographic image forming apparatuses such as copiers, printers, facsimile machines and complex machines of these, a toner image formed on an image bearing member such as a photoconductive drum having a photoconductive layer on the outer surface thereof is generally transferred to a surface of a transfer sheet.
In a general electrophotographic image forming apparatus, a charging process of uniformly charging the outer surface of the photoconductive drum, an exposing process for irradiating the charged outer surface of the photoconductive drum with a laser beam to form an electrostatic latent image, a developing process for attaching toner particles to the electrostatic latent image to form a toner image, a transferring process for transferring the toner image to a surface of a transfer sheet, a fixing process for fixing the toner image on the surface of the transfer sheet to the surface of the transfer sheet and a cleaning process for removing the toner residual on the outer surface of the photoconductive drum in the transferring process are repeated.
In the above transferring process, a bias transfer method using a transfer roller as a transfer member arranged to face the photoconductive drum is widely known. In this method, a nip portion is formed by pressing the transfer roller into contact with the photoconductive drum. The transfer roller applies a bias voltage to the transfer sheet from the rear side of the transfer sheet when the transfer sheet passes the nip portion. By giving transfer charges to the transfer sheet, the toner on the photoconductive drum is transferred to the transfer sheet.
At this time, an amount of electric charges necessary for the toner transfer is constant. Thus, in an image forming apparatus having a fast processing speed, transfer charges need to be given within a shorter period of time, wherefore a transfer roller needs to apply a higher transfer bias voltage.
On the other hand, in a pre-nip area upstream of the nip portion in a conveying direction, an electric field is generated between the transfer roller and the photoconductive drum by the bias voltage applied to the transfer roller. Accordingly, if a high-voltage transfer bias is applied, the intensity of the electric field generated in the pre-nip area becomes higher. Under such a high electric field, discharge occurs according to Paschen's law if the intensity of the electric field in a gap exceeds a discharge starting point.
Further, if discharge occurs between the transfer sheet and the photoconductive drum, electric charges having a polarity opposite to the charging polarity of the toner particles are given to the toner particles in the pre-nip area, whereby a charge amount distribution of the toner particles largely shifts toward the opposite polarity side. Such toner particles having the opposite polarity is unlikely to be transferred to the transfer sheet, and there has been a problem of occurrence of so-called hollow defects (white spots) phenomenon in which a part where discharge has occurred become white in the toner image transferred to the surface of the transfer sheet.
In view of this, it is thought to dispose a guide member for guiding the transfer sheet to the nip portion at a position immediately upstream of the nip portion in the conveying direction.
Concerning such a guide member, Japanese Unexamined Patent Publication No. 2004-115266 (D1) discloses the use of a pair of guide members located at the upper and lower sides of a conveyance path for a transfer sheet and inclined toward each other such that spacing therebetween becomes gradually narrower toward a downstream side in the conveying direction. The document D1 also discloses that the guide member located above the conveyance path out of the pair of guide members is formed with a cutout having less interference (sliding contact) during the passage of a postal card (thick sheet) or the like in order to soften a shock caused by the collision of the postal card with the nip portion.
Further, Japanese Unexamined Patent Publication No. 2001-117375 (D2) discloses that an elastic guide member is disposed below a conveyance path for a transfer sheet and the leading end of the guide member is located closer to a photoconductive drum than a tangent at a transfer nip. This construction is proposed to prevent hollow defects caused by discharge in a pre-nip area.
However, if the guide member located above the conveyance path is formed with the cutout as disclosed in the document D1, the influence of the guide member located below the conveyance path becomes stronger when the transfer sheet enters the nip portion. Thus, a sufficient effect of preventing hollow defects resulting from the occurrence of discharge in the pre-nip area cannot be expected.
Further, in the construction as disclosed in the document D2, a force of conveying the transfer sheet becomes weaker in the vicinity of a widthwise middle part of the transfer sheet due to the deflection of the transfer roller along the longitudinal direction of the transfer roller (width direction of the transfer sheet). Thus, a conveying speed is slower for narrow transfer sheets than for recording media having a maximum width. In addition, in the case of using a thick postal card or the like as the transfer sheet, a conveyance load of the guide member is larger than in the case of a PPC sheet as the transfer sheet, wherefore there is a problem of further slowing down the conveying speed.